Division of bit streams to produce spatial paths for multicarrier transmission

ABSTRACT

A device for bit-demultiplexing in a multicarrier MIMO communication system (e.g. precoded spatial multiplexing MIMO communication systems using adaptive OFDM), including a multicarrier MIMO transmitter and a multicarrier MIMO receiver. The multicarrier MIMO transmitter includes a demultiplexer and symbol mapper unit receiving an input bit stream and generating a plurality of symbol streams, each symbol stream being associated with a different transmission channel and including a plurality of data symbols, each data symbol being attributed to a different carrier; one or more multicarrier modulators generating at least two multicarrier modulated signals based on the symbol streams; and at least two transmit ports respectively transmitting the at least two multicarrier modulated signals, wherein a data throughput rate of each transmission channel is separately variable.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and is based upon and claims thebenefit of priority under 35 U.S.C. §120 for U.S. Ser. No. 13/254,126,filed Aug. 31, 2011 the entire contents of which are incorporated hereinby reference. U.S. Ser. No. 13/254,126 is the national stage ofPCT/EP10/50882 filed Jan. 27, 2010, and claims the benefit of priorityunder 35 U.S.C. §119 from European Patent Application No. 09156480.7,filed Mar. 27, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of bit-de-/multiplexing inmulticarrier MIMO communication systems (e.g. precoded spatialmultiplexing MIMO communication systems using adaptive OFDM). Thepresent invention especially relates to a multicarrier MIMO transmitterand a multicarrier MIMO receiver.

2. Description of the Prior Art

MIMO (multiple input multiple output) communication systems are wellknown in the art. A MIMO transmitter comprises at least two transmitports (e.g. antennas) and MIMO receiver comprises at least two receiveports (e.g. antennas). Signals are transmitted from the transmit portsto the receive ports via a communication channel which generally mixesthe signals transmitted by a plurality of transmit ports. The MIMOreceiver comprises a MIMO detector (MIMO decoder) which “demixes” thereceived signals and obtains the information comprised in the signalstransmitted by the MIMO transmitter. Various kinds of MIMO transmittersare known, e.g. space-time encoded MIMO transmitters, and spatialmultiplexing MIMO transmitters with precoding (e.g. eigenbeamformingMIMO transmitters) and without precoding. For each type of MIMOtransmitter, a corresponding MIMO receiver is employed. MIMO technologyachieves a higher spectral efficiency and higher link reliability. Also,multicarrier modulation schemes like, for example, OFDM (orthogonalfrequency division multiplexing) and multicarrier wavelet modulation arewell known. MIMO systems have two or more transmission channels. Usuallyan application has one data source and one data sink. Therefore, thedata to be transmitted have to be split (demultiplexed, demuxed) tovarious transmission channels. At the receiver, the split data receivedon individual transmission channels need to be combined (multiplexed,muxed) again. This task is called bit-de-/multiplexing for theindividual transmission channels. PLC (power line communication or powerline carrier) communication systems transmit data using one or moreconductors that are regularly used for electric power transmission.Wireless multicarrier MIMO systems use a constant symbol mapping (e.g.QAM mapping) for all carriers. This results in a constant throughput onthe individual channels, so the demultiplexing at the transmitter is astatic split of the incoming bits to the outgoing bits. At the receiver,the bits will be muxed in a static way again.

It is the object of the present invention to provide for improvedmulticarrier MIMO transmitters and receivers, especially to provide foran improved robustness of data transmission and/or reduced complexity ofdata transmission.

SUMMARY OF THE INVENTION

A multicarrier MIMO transmitter according to the present inventioncomprises a demultiplexer and symbol mapper unit for receiving an inputbit stream and generating a plurality of symbol streams, each symbolstream being associated with a different transmission channel andcomprising a plurality of data symbols, each data symbol beingattributed to a different carrier; one or more multicarrier modulatorsfor generating at least two multicarrier modulated signals based on thesymbol streams; and at least two transmit ports for respectivelytransmitting the at least two multicarrier modulated signals, wherein adata throughput rate of each transmission channel is separatelyvariable.

Because the data throughput rate of each transmission channel isseparately varied, the data throughput rate can be adapted e.g.according to channel conditions, which provides for a more robust andmore efficient data transmission.

Advantageously, at least one data symbol represents a non-continuousarrangement of bits from the input bit stream.

Advantageously, the demultiplexer and symbol mapper unit is adapted tosplit the input bit stream into a plurality of split bit streams and togenerate each of said symbol streams based on a different one of saidsplit bit streams.

Advantageously, each transmission channel has allocated a number of bitsrepresenting the data throughput rate of the respective transmissionchannel and said splitting of the input bit stream into the plurality ofsplit bit streams is based on the number of bits allocated to thetransmission channels and/or is based on one or more ratios of thenumber of bits allocated to the transmission channels.

Advantageously, the bits of the split bit streams are evenly distributedwithin the input bit stream. Of course, this is not a property of theinput bit stream but a property of the employed multiplexing. In otherwords, each split bit stream is seen as a group of bits and thedifferent groups of bits are evenly distributed in the input bit stream.In still other words, each split bit stream is seen as a group of bitsand said splitting of the input bit stream into the split bit streams isperformed in a way that the members of the groups of bits are evenlydistributed in the input bit stream. In still other words, each bit ofthe input bit stream is seen as corresponding to a class, the respectiveclass being given by the split bit stream of which the bit is part of,and the classes are evenly distributed in the input bit stream.

Advantageously, the input stream comprises at least two sections, eachsection comprising at least two groups of bits, each group of the atleast two groups being demultiplexed to another one of the split bitstreams and being given by one or more consecutive bits.

Alternatively, each data symbol advantageously represents a continuoussequence of bits from the input bit stream.

Advantageously, demultiplexer and symbol mapper unit is adapted to mapthe bits of the input data bit stream to the data symbols anddemultiplex the data symbols to the plurality of symbol streams.

Advantageously, a group of all data symbols of a symbol streamrepresents a continuous sequence of bits from the input bit stream; or agroup of all data symbols of corresponding subcarriers of thetransmission channels represents a continuous sequence of bits from theinput bit stream; or each of the continuous sequences of bits of theinput bit stream is transmitted on a specific subcarrier of a specifictransmission channel defined by a pseudo random sequence.

Advantageously, the demultiplexer and symbol mapper unit is adapted tosequentially map the continuous bit sequences to data symbols.

Advantageously, a constellation used in the generation of data symbolsis adapted for at least some subcarriers on at least some transmissionchannels.

A multicarrier MIMO receiver according to the present inventioncomprises at least two receive ports for respectively receiving at leasttwo multicarrier modulated signals; one or more multicarrierdemodulators for demodulating the received at least two signals, adetector for generating at least two symbol streams based on thedemodulated at least two signals, each symbol stream being associatedwith a different transmission channel and comprising a plurality of datasymbols; and a symbol demapper and multiplexer unit for generating anoutput bit stream based on the at least two symbol streams.

Advantageously, at least one data symbol represents a non-continuousarrangement of bits of the output bit stream.

Advantageously, the symbol demapper and multiplexer unit is adapted todemap the plurality of symbol streams into a corresponding plurality ofsplit bit streams.

Advantageously, wherein each transmission channel has allocated a numberof bits indicating the data throughput rate of the respectivetransmission channel and the symbol demapper and multiplexer unit isadapted to multiplex said split bit streams into said output bit streambased on the number of bits allocated to the transmission channelsand/or based on one or more ratios of the number of bits allocated tothe transmission channels.

Advantageously, bits of the split bit streams are evenly distributedwithin the output bit stream.

Advantageously, wherein the output bit stream comprises at least twosections, each section comprising at least two groups of bits, eachgroup of the at least two groups being given by one or more consecutivebits and being multiplexed into the output bit stream from another oneof the split bit streams.

Alternatively, each data symbol advantageously represents a continuoussequence of bits from the output bit stream.

Advantageously, the symbol demapper and multiplexer unit is adapted tomultiplex the data symbols of the plurality of symbol streams into asingle symbol stream and demap the multiplexed data symbols into theoutput bit stream.

Advantageously, a group of all data symbols of one of the plurality ofsymbol streams represents a continuous sequence of bits from the outputbit stream; or a group of all data symbols of corresponding subcarriersof the transmission channels represents a continuous sequence of theoutput bit stream; or each of the continuous sequences of bits of theoutput bit stream is received on a specific subcarrier of a specifictransmission channel defined by a pseudo random sequence.

Advantageously, the symbol demapper and multiplexer unit is adapted tosequentially demap the data symbols to the continuous bit sequences.

Advantageously, a constellation used in the generation of the output bitstream based on the at least two symbol streams is adapted for at leastsome subcarriers on at least some transmission channels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of an embodiment of thetransmitter according to the present invention.

FIG. 2 shows schematic representation of an embodiment of the receiveraccording to the present invention.

FIG. 3 shows a schematic representation of a first alternativeembodiment of the demultiplexer and symbol mapper unit.

FIG. 4 shows a schematic representation of a first alternativeembodiment of the symbol demapper und multiplexer unit.

FIG. 5 shows a flow diagram of a bit-de-/multiplexing algorithmaccording to the first alternative embodiment.

FIG. 6 shows a first example of the bit-de-/multiplexing obtained usingthe algorithm.

FIG. 7 shows a second example of the bit-de-/multiplexing obtained usingthe algorithm.

FIG. 8 shows a schematic representation of a second alternativeembodiment of the demultiplexer and symbol mapper unit.

FIG. 9 shows a schematic representation of a second alternativeembodiment of the symbol demapper und multiplexer unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 and FIG. 2 show a first embodiment of the multicarrier MIMOtransmitter 1 and the multicarrier MIMO receiver 2 of the presentinvention. The transmitter 1 and the receiver 2 may be (or may becomprised in) any kind of communication devices like, as non limitingexamples, mobile phones, personal computers, televisions, audio and/orvideo recording and/or playback devices and PLC modems.

According to the present invention, the transmitter 1 comprises ademultiplexer and symbol mapper unit 6, one or more multicarriermodulators 20-1 to 20-n and at least two transmit ports 24-1 to 24-n.

The transmitter 1 of this embodiment further comprises an encoder 5which encodes an input bit stream according to an error correction codeand/or error detection code. As is known in the art, error correctioncodes are used for forward error correction. Both the encoded and theunencoded input bit stream will be called input bit stream in thefollowing.

The (encoded) input bit stream is then further processed by thedemultiplexer and symbol mapper unit 6. The demultiplexer and symbolmapper unit 6 maps the bits to data symbols by one or more symbolmappers (e.g. OFDM modulators) 10, 10-1 to 10-n as shown in FIG. 3 orFIG. 8. Hereby, the demultiplexer and symbol mapper unit 6 generates aplurality of symbol streams. Each symbol stream comprises a plurality ofdata symbols, each data symbol being attributed to (transmitted on) adifferent subcarrier. Each one of the symbol streams corresponds to adifferent MIMO transmission channel. The MIMO transmission channels mayalso be called MIMO paths. The number of MIMO transmission channels istypically the same as the number of transmit ports 24-1 to 24-n but mayalso be smaller than the number of transmit ports 24-1 to 24-n. Themapping of bits to data symbols is performed according to constellationsset for each subcarrier on each transmission channel. The constellationsare varied according to channel conditions (adaptive multicarriermodulation). For example, the constellation of a subcarrier of atransmission channel may be selected according to a channel qualitymeasure (e.g. signal-to-noise ratio, SNR) determined for this subcarrieron this transmission channel. The constellations may be selected foreach subcarrier on each transmission channel separately. For examplenoise, frequency dependent attenuation, changes in the MIMO channel(e.g. by switching a light on/off in a PLC channel) can influence orchange the selection of a constellation and thereby influence or changethe number or ratio (proportion) of bits allocated to the transmissionchannels. Constellation information indicating the constellations toemploy by the one or more symbol mappers may, for example, be stored ina memory 14 of the transmitter 1. The constellation information issometimes called tone map (e.g. OFDM tone map). Adaptive multicarriermodulation is especially advantageous in case of PLC communicationsystems. The data throughput varies with the (size of the)constellation. The size of a constellation is the amount of information(e.g. the number of bits) that is comprised in data symbol obtainedaccording to the constellation. The size of the constellation istherefore the amount of information (e.g. number of bits) that istransmitted on the subcarrier of the transmission channel. Thus, thepresent invention provides bit-de-/multiplexing with a variablethroughput rate. The throughput rate of each transmission channel variesseparately. The demultiplexing of the data to the transmission channelsmay be performed on bit level or on data symbol level as will bedescribed below in more detail.

The symbol streams are then MIMO precoded by a precoder 18. Theprecoding employed for example may be an eigenbeamforming precoding.However, any suitable MIMO precoding might be employed. For example,precoding according to a space-time code (e.g. Alamouti code) might beemployed. The precoder might also be omitted. In the case where theprecoder is omitted, the transmission channels correspond to thetransmit ports, that is, each symbol stream is transmitted by anotherone of the transmit ports 24-1 to 24-n. On the other hand, when forexample eigenbeamforming is employed, the transmission channels do notcorrespond to the transmit ports 24-1 to 24-n. This is becauseEigenbeamforming “mixes” the input symbol stream to generate theprecoded (output) symbol streams. The precoder 18 generates a pluralityof precoded data symbol streams according to the number of transmitports 24-1 to 24-n.

Each precoded data symbol stream is then multicarrier modulated (e.g.OFDM modulated, multicarrier wavelet modulated) by a correspondingmulticarrier modulator (e.g. OFDM modulator) 20-1 to 20-n andtransmitted by a corresponding one of the transmit ports 24-1 to 24-n.Alternatively, a single multicarrier modulator sequentially modulatesthe precoded data symbols. In this case, the modulated data symbolstreams are then demultiplexed to the respective transmit ports 24-1 to24-n. The transmit ports 24-1 to 24-n might for example be antennas. Inone embodiment however, the transmit ports 24-1 to 24-n are adapted totransmit signals to the receiver 2 via two or more conductors. Hereby,the conductors may be dedicated data transition lines ormay—additionally or primarily—serve the purpose of transmittingelectrical power (e.g. PLC transmitter transmitting on mains powerlines).

The receiver 2 receives the signals transmitted on the MIMO channel (notshown) by the transmitter 1 with its at least two receive ports 30-1 to30-n. The signals comprise the information of the input bit stream. Thereceive ports 30-1 to 30-n may for example be antennas. In oneembodiment however, the receive ports 30-1 to 30-n are adapted toreceive the signals via two or more conductors. The conductors mayhereby be dedicated to data transmission or may—additionally orprimarily—serve the purpose of electric power transmission (e.g. PLCreceiver receiving on mains power lines).

The at least two multicarrier modulated signals are then demodulated byrespective at least two multicarrier demodulators (e.g. OFDMdemodulators, multicarrier wavelet demodulators) 34-1 to 34-n. Thedemodulation may also be done sequentially, as 20-1 to 20-n intransmitter 1. Instead of a plurality of multicarrier demodulatorsaccording to the number of receive ports 30-1 to 30-n, also lessmulticarrier demodulators, e.g. a single multicarrier demodulator, mightbe employed. In this case, the received multicarrier modulated signalsare, at least partially, demodulated in sequence.

A channel estimator 35 obtains channel state information (CSI) based onthe demodulated signals. The CSI might for example comprise a channelmatrix and a channel quality measure (e.g. signal-to-noise ratio) foreach subcarrier on each transmission channel. Determination of channelstate information might for example be based on training symbols and/orpilot symbols. As described above, the constellations used in the symbolmapping and demapping are adapted to channel conditions. The specificconstellations to employ may be determined based on the channel stateinformation. For example, the constellation for a specific subcarrier ona specific transmission channel might be determined based on the channelquality measure (e.g. signal-to-noise ratio) of the specific subcarrierof the specific transmission channel. The CSI and the constellationinformation might, for example, be stored in a memory 44 of the receiver2. Thus, the present invention provides bit-de-/multiplexing with avariable throughput rate.

A detector 36 performs a MIMO detection (MIMO decoding) on thedemodulated signals. The detection may be based on CSI obtained by thechannel estimator 35. Any kind of detectors (decoders) might beemployed. For example, zero forcing (ZF), minimum mean square error(MMSE) and maximum likelihood (ML) detectors might be employed. As aresult of the MIMO detection, at least two symbol streams correspondingto the at least two transmission channels are obtained.

In the symbol demapper and multiplexer unit 38, the detected symbolstreams are processed reversing the processing of the demultiplexer andsymbol mapper unit 6 of the transmitter 1 in order to obtain an outputbit stream. Especially, each data symbol comprised in the symbol streamsis mapped (“demapped”) to a number of bits represented by the datasymbol by one or more symbol demappers 40, 40-1 to 40-n as shown in FIG.4 or FIG. 9. As described above, adaptive multicarrier demodulationmight be employed. The constellation information indicating theconstellations to use might for example be stored in a memory 44 of thereceiver 2. The multiplexing of the data received on the plurality oftransmission channels into the output bit stream may be performed on bitlevel or on data symbol level as will be described below in more detail.The output bit stream is still encoded with the encoding applied by theencoder 5 of the transmitter 1.

The encoded output bit stream is decoded by a decoder 48 by an errorcorrection and/or detection method corresponding to the error correctionand/or detection code employed by the encoder 5. As a result, a decodedoutput bit stream is obtained. The encoded and the decoded output bitstream will in the following both be referred to as output bit stream.

The receiver 2 may further comprise a transmitting section 46 and thetransmitter 1 may further comprise a receiving section 16. By means ofthe transmitting section 46 and the receiving section 16 a back channelis provided, by which any kind of information can be transmitted fromthe receiver 2 to the transmitter 1. The transmitting section 46 may,but need not, have the structure and the functionality of thetransmitter 1. Using the transmitting section 46, the receiver 2 may,for example transmit channel state information, precoding informationand constellation information (e.g. OFDM tonemap), that is, informationindicating the constellations to use in the mapping of bits to datasymbols for each subcarrier on each transmission channel. The receivingsection 16 may, but need not, have the structure and the functionalityof the receiver 2. Using the receiving section, the transmitter 1 may,for example, receive the channel state information, precodinginformation and the constellation information generated and transmittedby the receiver 2.

Now, two alternative embodiments of the transmitter 1 and the receiver 2will be described. In the first alternative embodiment,bit-de-/multiplexing is performed on the bit level. In the secondalternative embodiment, bit-de-/multiplexing is performed on the symbollevel. Both the first and the second alternative embodiments have thestructure and operation as described above in relation to FIGS. 1 and 2.

FIGS. 3 and 4 show the demultiplexer and symbol mapper unit 6 of thetransmitter 1 and the symbol demapper and multiplexer unit 38 of thereceiver 2 according to the first alternative embodiment.

In the first alternative embodiment, the demultiplexer and symbol mapperunit 6 comprises a demultiplexer 8 operating on the bit level and aplurality of symbol mappers (e.g. QAM modulators) 10-1 to 10-n, eachsymbol mapper corresponding to a different MIMO transmission channel.The demultiplexer 8 demultiplexes the input bit stream into a number ofsplit bit streams. The split bit streams are processed in parallel andthe input bit stream is clocked or, at least, is clockable at a higherrate than any of the split bit streams. The number of split bit streamsis at least two and is given by the number of MIMO transmissionchannels. Each split bit stream corresponds to a different one of thetransmission channels. The bits are represented by small rectangles.Bits represented by a diagonally striped rectangles are demuxed to thefirst transmission channel. Bits represented by unfilled rectangles aredemuxed to the n-th transmission channel. It is noted that the depictedsection of the input bit stream does not comprise bits demuxed to othertransmission channels than the first transmission channel and the n-thtransmission channel. This is for purpose of illustration only andshould not be construed as limiting. Generally there will be bitsdemuxed to transmission channels other than the first transmissionchannel and the n-th transmission channel interspersed in the input bitstream. Each split bit stream is then mapped to a corresponding symbolstream by a corresponding symbol mapper 10-1 to 10-n. Thus, the varioussplit bit streams are processed (mapped) in parallel. As describedabove, variable constellations (adaptive multicarrier modulation) areemployed. Advantageously, an equal distributed bit-demultiplexing isemployed, which guarantees a balanced distribution of the bits to thetwo or more bit streams. This will be described in more detail below.

Similarly, the symbol demapper and multiplexer unit 38 of the firstalternative embodiment comprises a number of symbol demappers 40-1 to40-n according to the number of transmission channels (symbol streams)and a multiplexer 42 operating on the bit level. The plurality of symbolstreams feed into the symbol demapper and multiplexer unit 38 areprocessed in parallel. Each one of the symbol demappers 40-1 to 40-nmaps (“demaps”) the data symbols of a different one of the symbolstreams to a corresponding split bit stream. The split bit streamsobtained by demapping the symbol streams are then multiplexed into asingle bit stream, which is the output bit stream, by the multiplexer42. The output bit stream is clocked or, at least, is clockable athigher rate than any of the split bit streams.

A specific embodiment of the bit level bit-de-/multiplexing applied inthe transmitter 1 and the receiver 2 of the first alternative embodimentis now explained with reference to FIG. 5. The algorithm is performedonce for each multicarrier symbol (e.g. OFDM symbol) or is performed atleast each time a constellation size used in the symbolmapping/demapping changes. The algorithm operates bit-wise. In thetransmitter 1, the algorithm determines which bit of the (encoded) inputbit stream is to be transmitted on which transmission channel (and,implicitly, on which subcarrier), so that the split bit streams can beproperly demultiplexed from the input bit stream. In the receiver 2, thealgorithm determines which bit of the (encoded) output bit stream hasbeen transmitted on which transmission channel (and, implicitly, onwhich subcarrier), so that the split bit streams can be properlymultiplexed into the output bit stream. In the embodiment, the number oftransmission channels is assumed to be two. Some of the actionsperformed according to the algorithm as described below relate to thetransmitter 1 only. The skilled person will however recognize whatcorresponding actions have to be performed in the receiver 2.

In step S2 the number of bits allocated to (transmitted by) thetransmission channels are determined. The transmission channel with thehigher number of bits allocated is set path_b and the transmissionchannel with the lower number of bits allocated is set path_a.

In step S4 it is determined if the number of bits allocated to thetransmission channel path_a is zero. If yes, a number of consecutivebits from the input bit stream, which is given by the number of bitsallocated to path_b is allocated to path_b in step S8. If no, theprocess proceeds to step S10.

In step S10 the ratio of the bits allocated to the transmission channelpath_b and transmission channel path_a is determined.

In steps S12 and S14 the number of bits transmitted the plurality oftransmission channels is obtained and variables n, a_index and b_indexare set to one.

In step S16 it is determined if the variable n is larger than the totalnumber of bits allocated to the plurality of transmission channels. Ifyes, all bits have been allocated for the multicarrier symbol and thealgorithm is finished. If no, the algorithm proceeds to step S18.

In step S18 the inequality

n<a_index*(ratio+1)−ratio/2  (1)

is evaluated. The second term “ratio/2” might be omitted or replaced byanother constant. Constant here means independent of the variable n andnot variable during the algorithm. In case the inequality holds, thealgorithm proceeds to step S20. In case the inequality does not hold,the algorithm proceeds to step S26.

In step S20 it is determined if there are still unallocated bits ontransmission channel path_b. If yes, the n-th bit of the input bitstream is allocated to transmission channel path_b and the variableb_index is augmented by one in step S22 and the algorithm proceeds tostep S30. If no, the algorithm proceeds to step S24.

In step S24 the n-th bit of the input bit stream is allocated totransmission channel path_a and the variable a_index is augmented by oneand the algorithm proceeds to step S30.

In step S26 it is determined if there are still unallocated bits ontransmission channel path_a. If yes, the n-th bit of the input bitstream is allocated to transmission channel path_a and the variablea_index is augmented by one in step S24 and the algorithm proceeds tostep S30. If no, the algorithm proceeds to step S28.

In step S28 the n-th bit of the input bit stream is allocated totransmission channel path_b and the variable b_index is augmented byone. And the algorithm proceeds to step S30.

In step S30 the variable n is augmented by one and the algorithm returnsto step S16.

Thus, the description of the algorithm as operated by the transmitter 1is finished. When executed in the receiver 2, the steps S4, S22 and S24and S28 take on the following form:

In step S4, it is determined if the number of bits allocated to thetransmission channel path_a is zero. If yes, the output bit stream isgiven by the bit stream received on transmission channel path_b. If no,the process proceeds to step S10.

In step S22, the n-th bit of the output bit stream is taken fromtransmission channel path_b (n-th bit of the output bit stream is givenby the next unallocated bit of transmission channel path_b), thevariable b_index is augmented by one and the algorithm proceeds to stepS30.

In step S24, the n-th bit of the output bit stream is taken fromtransmission channel path_a (n-th bit of the output bit stream is givenby the next unallocated bit of transmission channel path_a), thevariable a_index is augmented by one and the algorithm proceeds to stepS30.

In step S28, the n-th bit of the output bit stream is taken fromtransmission channel path_b (n-th bit of the output bit stream is givenby the next unallocated bit of transmission channel path_b), thevariable b_index is augmented by one and the algorithm proceeds to stepS30.

It is clear that, steps which are identical for the transmitter 1 andthe receiver 2 need not be performed twice. Results and intermediatedata may be shared (transmitted) between the transmitter 1 and thereceiver 2 so as to reduce the complexity of computation.

Examples of the bit-de-/multiplexing obtained by this algorithm aredepicted in FIGS. 6 and 7.

FIG. 6 shows the input/output bit stream, the split bit streamcorresponding to transmission channel path_b, the split bit streamcorresponding to transmission channel path_a and the input/outputbitstream represented by the bits of the split bit streams in the casewhere the ratio of bits allocated to transmission channel path_b andtransmission channel path_a is three (i.e. ratio=3). The input bitstream is given by a sequence of bits N1, N2, N3, . . . . The split bitstream corresponding to a transmission channel path_a is given by asequence of bits A1, A2, A3, . . . . The split bit stream correspondingto a transmission channel path_b is given by a sequence of bits B1, B2,B3, . . . . The first, the second and the third bit B1, B2 and B3 of thesplit bit stream of path_b are given by the first, the second and thefourth bit N1, N2 and N4 of the input bit stream. Correspondingly, thefirst four bits N1 to N4 of the output bit stream are given by the bitsB1, B2, A1 and B3, respectively. The pattern of allocation BBAB(equivalently BABB, ABBB) repeats every four bits. In an example wherethe first subcarrier of path_b is 16-QAM modulated, bits N1,N2, N4 andN5 are mapped to a 16-QAM modulated data symbol (“first data symbol”).Since the bit N3 is not part of the bits represented by the first datasymbol, the first data symbol represents a non-continuous arrangement(non continuous sequence) of bits of the input/output bit stream. In anexample where the first subcarrier of path_a is QPSK modulated, bits N3and N7 are mapped to a QPSK modulated symbol (“second data symbol”).Since the bits N4 to N6 are not part of the bits represented by thesecond data symbol, the second data symbol represents a non-continuousarrangement (non-continuous sequence) of bits from the input/output bitstream. Of course, the split bit streams are processed (e.g. mapped) inparallel as described above. FIG. 5 shall NOT be understood as if therewere a ordering (e.g. time ordering) BETWEEN the bits of the differenttransmission channels (e.g. it is NOT implied that the bit B2 is mappedto a data symbol before the bit A1 is mapped to a data symbol). Thehorizontal direction of FIG. 5 solely represents the order WITHIN therespective streams.

FIG. 7 shows the same bit streams with the same representations as FIG.6 in the case where the ratio of allocation is 3/2. In this case it canbe seen that the pattern of allocation BABBA (equivalently ABBAB, BBABA,BABAB, ABABB) is repeated every five bits.

As can be seen from FIGS. 6 and 7, when the input/output bit stream isrepresented by the bits of the split bit streams of transmission channelpath_b and path_a, the bits of the split bit streams are evenly spread(evenly distributed, uniformly distributed) within the input/output bitstream. No undue accumulation of the bits of a given split bit streamoccurs in the input/output bit stream. Thus, consecutive bits of theinput/output bit stream are maximally spread to the transmissionchannels. This improves the reliability of data transmission andprovides advantageous conditions for successful operation of the(forward) error correcting and/or error detecting code. This is becausethe number of burst errors and/or the length of burst errors in thereceived encoded bit stream is reduced.

As can be seen, the input/output bit stream, when represented by thebits of the split bit streams, is of a structure comprising at least twosections (e.g. A1B3B4B5 and A2B6B7B8 in the case of ratio=3 or B1A1 andB2B3A2 in the case of ratio=3/2), each section comprising at least twogroups of bits, each group of the at least two groups of bits beingdemultiplexed to another one of the split bit streams (e.g. A1multiplexed to path_a, B3B4B5 multiplexed to path B in the case ofratio=3 or B1 multiplexed to path_b and A1 multiplexed to path_a). Thenumber of the groups, as in these examples, advantageously is the sameas the number of transmission channels.

As can be seen, the input/output bit stream, when represented by thebits of the split bit streams, is of a structure comprising at least twosections (e.g. B1B2A1B3 and B4B5A2B6 in the case of ratio=3 orB1A1B2B3A2 and B4A3B5B6A4 in the case of ratio=3/2), whereby eachsection comprises bits of split bit streams in the same ratio(proportion) as the ratio (proportion) of the bits of the split bitstreams comprised the input/output bit stream. While the algorithmgenerally produces such a structure, there exist ratios of allocationwhere such structure is not feasible due to mathematical impossibility.In this case the algorithm produces only one section that comprises thebits of the split bit streams in the same ratio as the input/output bitstream. This one structure is the input/output stream itself.

FIGS. 8 and 9 show the demultiplexer and symbol mapper unit 6 of thetransmitter 1 and the symbol demapper and multiplexer unit 38 of thereceiver 2 according to the second alternative embodiment in which thebit-de-/multiplexing is performed on symbol level.

When operating on the data symbol level, the complexity ofdemultiplexing and multiplexing can be greatly reduced. To achieve this,a symbol mapper 10 (e.g. QAM modulator) is provided which has athroughput that is sufficient for all transmission channels in sum sothat it sequentially maps the bits of the input data stream to datasymbols for all subcarriers on all transmission paths. The obtained datasymbols are then demultiplexed by a demultiplexer 9 to the at least twotransmission channels.

In some embodiments a feedback control signal from the symbol mapper 10to the encoder 5 might be provided to adjust the data throughput (e.g.the symbol mapper requests the necessary number of bits from the encoder5).

The symbol mapper 10 may map the subcarriers of the differenttransmission channels in a block-wise way, a sequential way or apseudorandom way for example.

In the block-wise way, the symbol mapper 10 first maps all subcarriersof the first transmission channel then maps all subcarriers of thesecond transmission channel, then maps all subcarriers of the thirdtransmission channel and so on until all subcarriers of all transmissionchannels have been mapped (e.g. QAM modulated).

In the sequential way, the symbol mapper 10 first maps the firstsubcarriers for all paths, then maps the second subcarriers for allpaths, then maps the third subcarriers for all paths and so on until allsubcarriers of all paths are mapped (e.g. QAM modulated).

It is understood that subcarriers are normally ordered according to aphysical characteristic (e.g. frequency of OFDM subcarrier, bandwidth ofcarrier wavelet). This natural order is the order in which thesubcarriers are mapped in the block-wise way and the sequential way. Ofcourse each subcarrier is represented once on each transmission channeland there is a block-wise and a sequential natural order of thecombinations of the subcarriers with the transmission channels. In theblock-wise natural order, the combinations of subcarriers withtransmission channels are first grouped according to transmissionchannels and then, within each group, according to the subcarriernatural order. In the sequential natural order, the combinations ofsubcarriers with transmission channels are first grouped according tothe natural order of subcarriers and then, within each group, accordingto transmission channels.

In the pseudorandom way, the symbol mapper 10 maps (e.g. QAM modulates)the subcarriers of all transmission channels according to a pseudorandomsequence. When the number of transmission channels is t and the numberof subcarriers is c, the length of the sequence is t*c. With eachsubcarrier of each transmission channel there is associated a digit ofthe pseudorandom sequence. The k-th digit of the random sequenceindicates which subcarrier and transmission channel combinationaccording to the natural order will be mapped at the k-th step. In otherwords, the pseudorandom sequence will be read sequentially, 1^(st),2^(nd) k-th . . . until (t*c)-th and at the same time the incomingbit-stream will be allocated to the subcarrier with the index defined bythe pseudorandom sequence. When the k-th digit of the sequence is j, thecurrent bits will be mapped to the j-th combination of subcarrier andtransmission channel according to the natural order. For example, whenj=1, the current bits of the input bit stream will be transmitted on thej-th subcarrier and transmission channel combination. The pseudorandomsequence can for example be saved in the memory 14. The demultiplexerand symbol mapper unit 6 may then read the sequence from the memory 14and perform the corresponding reordering (carrier index reordering). Insome embodiments, the demultiplexer and symbol mapper unit 6 and theprecoder 18 may perform the reordering collaboratively. In this case,the precoder 18 operates based on pseudorandom sequence. For notchedcarriers (i.e. carriers on which no information is transmitted e.g.because the channel conditions are bad), the reordering remainsuninfluenced because the notch information is already included in theconstellation information.

Again, the symbol demapper and multiplexer unit 38 of the receiver 2performs the necessary operations to restore the original bit sequence(i.e. reverses the operation of the demultiplexer and symbol mapper unit6 of the transmitter 1). To this effect, the symbol demapper andmultiplexer unit 38 may for example comprise a multiplexer 43 operatingon the symbol level and a single symbol demapper 40 as is depicted inFIG. 9. The multiplexer 43 multiplexes the symbols received on the atleast two transmission channels into a single symbol stream and providesit to the symbol demapper 40. The symbol demapper (e.g. QAM demodulator)40 has a throughput that is sufficient for all transmission channels insum and generates the output bit stream by sequentially demapping (e.g.QAM demodulating) all subcarriers on all transmission channels. Thesymbol demapper and multiplexer unit 38 may for example operateaccording to the principles of the block-wise way, the sequential wayand the pseudorandom way described above. In case of the pseudorandomway, the symbol demapper and multiplexer unit 38 may read thepseudorandom sequence stored in memory 44 and perform a correspondingreordering of the received data symbols, so that the original input bitstream can be restored. In some embodiments, the symbol demapper andmultiplexer unit 38 and the MIMO decoder 35 may perform the reorderingcollaboratively. In this case, the decoder 35 operates based onpseudorandom sequence.

The pseudorandom sequence may be fixed or may be different for differentmulticarrier symbols (e.g. OFDM symbols). When the pseudorandom sequenceis fixed, the corresponding ordering of the subcarrier and transmissionchannels can be hardcoded into the transmitter 1 and the receiver 2,which reduces complexity of the devices.

The carrier reordering according to the pseudorandom sequence improvesthe robustness of data transition and provides favorable conditions forsuccessful operation of the error correction and/or detection code.

Between the demultiplexer and symbol mapper unit 6 and the precoder 18,an additional interleaver (not shown) can be provided. The interleavermight for example scramble (exchange) information between all, or atleast some, subcarriers and transmission channels with identicalconstellations. This further increases the robustness of datatransmission.

Bit-de-/multiplexing on the data symbol level reduces the complexity ofthe de-/multiplexer units as compared to bit-de-/multiplexing on bitlevel. In the most complex case of data symbol levelbit-de/multiplexing, still only a carrier index reordering algorithm hasto be applied as described above. Further, as was shown above, a singlesymbol mapper and a single symbol demapper can be employed in the caseof symbol level bit-de-/multiplexing, which reduces further thecomplexity of the transmitter 1 and the receiver 2.

It is noted that the same transmission signals as obtained by the secondalternative embodiment can also be obtained with the structure of thefirst alternative embodiment depicted in FIGS. 3 and 4. This is possiblesince the structure of the first alternative embodiment is able toperform a more fine grained (i.e. bit level) bit-de-/multiplexing thanthe structure of the second alternative embodiment, which performs asymbol level bit-de-/multiplexing. Thus, also the demultiplexer andsymbol mapper unit with the structure of FIG. 3 and the symbol demapperand multiplexer unit with the structure of FIG. 4 might operateaccording to the principles of the block-wise way, the sequential wayand the pseudorandom way described above. In this case however, thedescribed benefit of reduced complexity is not fully obtained. Also, insome embodiments, feedback control signals would be required from eachof the symbol mappers 10-1 to 10-n and the demultiplexer 8 to theencoder 5 to adjust the data throughput.

Instead of the single encoder 5 also a plurality of (error correctionand/detection code) encoders, one for each transmission channel, mightbe provided between the demultiplexer and mapper unit 6 and the precoder18. In this case, a plurality of (error correction and/or detectioncode) decoders might be provided, one decoder for each transmissionchannel, between the detector 36 and the multiplexer and demapper unit38.

Instead of the single encoder 5 also a plurality of (error correctionand/detection code) encoders, one for each transmission channel, mightbe provided between the demultiplexer 8 and the symbol mappers 10-1 to10-n. In this case, a plurality of (error correction and/or detectioncode) decoders might be provided, one decoder for each transmissionchannel, between the symbol demappers 40-1 to 40-n and the multiplexer42.

While having explained embodiments of the present invention, where achannel estimation is performed at the receiver side, the presentinvention is not limited to this and channel estimation might also beperformed at the transmitter side (e.g. in case of a symmetric MIMOchannel).

1. A PLC device adapted for transmitting data symbols on a plurality ofsub-carriers via a first transmission channel and a second transmissionchannel, comprising: a demultiplexer unit configured to receive an inputbit stream and to provide a first and a second split bit stream, whereincontinuous sequences of consecutive bits of the input bit stream arealternately allocated to the first and the second split bit streams; afirst symbol mapper unit configured to map bits included in the firstsplit bit stream to first data symbols for the sub-carriers of the firsttransmission channel and a second symbol mapper unit configured to mapbits included in the second split bit stream to second data symbols forthe sub-carriers of the second transmission channel, wherein each of thefirst data symbols represents a continuous sequence of bits from theinput bit stream, each of the second data symbols represents acontinuous sequence of bits from the input bit stream, and first andsecond data symbols assigned to a corresponding sub-carrier of the firstand second transmission channels represent a continuous bit sequencefrom the input bit stream.
 2. The PLC device according to claim 1,wherein the first transmission channel is defined by a first combinationof wires and the second transmission channel is defined by a secondcombination of wires of a powerline network.
 3. The PLC device accordingto claim 1, wherein the sequences have variable lengths.
 4. The PLCdevice according to claim 1, wherein the demultiplexer unit isconfigured to determine a number of bits allocated to the split beamstreams according to constellation information indicating constellationsused by the symbol mapper units.
 5. The PLC device according to claim 4,wherein the constellations are determined based on a channel qualitymeasure.
 6. The PLC device according to claim 1, wherein the symbolmapper units are configured to map the split bit streams in dependenceon a channel quality measure of the specific sub-carrier of the specifictransmission channel.
 7. The PLC device according to claim 1, comprisinga precoder unit configured to apply a MIMO precoding scheme on a firstsymbol stream including the first data symbols and on a second symbolstream including the second data symbols.
 8. The PLC device according toclaim 7, comprising two multicarrier modulators configured to generateat least two multicarrier modulated signals based on the MIMO precodedsymbol streams.
 9. A PLC device adapted for receiving data symbols on aplurality of sub-carriers via a first transmission channel and a secondtransmission channel, comprising a first symbol demapper unit configuredto demap first data symbols transmitted over the first transmissionchannel to a first bit stream and a second symbol demapper unitconfigured to demap second data symbols transmitted over the secondtransmission channel to a second bit stream, and a multiplexer unitconfigured to multiplex the first and second bit streams to obtain asingle output bit stream, wherein each of the first data symbolsrepresents a continuous sequence of bits of the output bit stream, eachof the second data symbols represents a continuous sequence of bits ofthe output bit stream, and first and second data symbols assigned to acorresponding sub-carrier of the first and the second transmissionchannels represent a continuous bit sequence of the output bit stream.10. A method of transmitting data symbols on a plurality of sub-carriersvia a first transmission channel and a second transmission channel of aPLC system, comprising: providing, at a transmitting device, a first anda second split bit stream from an input bit stream, wherein continuoussequences of consecutive bits of the input bit stream are alternatelyallocated to the first and the second split bit streams; mapping, at thetransmitting device, bits included in the first split bit stream tofirst data symbols for the sub-carriers of the first transmissionchannel and mapping bits included in the second split bit stream tosecond data symbols for the sub-carriers of the second transmissionchannel, wherein each of the first data symbols represents a continuoussequence of bits from the input bit stream, each of the second datasymbols represents a continuous sequence of bits from the input bitstream, and first and second data symbols assigned to a correspondingsub-carrier of the first and the second transmission channels representa continuous bit sequence from the input bit stream.
 11. The methodaccording to claim 10, comprising: determining a number of bitsallocated to the split beam streams according to constellationinformation indicating constellations used for the mapping.
 12. Themethod according to claim 11, wherein the constellations are determinedbased on a channel quality measure.
 13. The method according to claim10, wherein the split bit streams are mapped in dependence on a channelquality measure of the specific sub-carrier of the specific transmissionchannel.
 14. The method according to claim 10, comprising MIMO precodinga first symbol stream including the first data symbols and a secondsymbol stream including the second data symbols.
 15. The methodaccording to claim 14, comprising generating at least two multicarriermodulated signals based on the MIMO precoded symbol streams.